Resistance fuse wire



March 31, 1970 v. D. ESTES RESISTANCE FUSE WIRE 2 Sheets-Sheet 1 Original Filed Oct. 4, 1965 Fig.3

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March 31, 1970 V. D. ESTES RESISTANCE FUSE WIRE Original Filed Oct. 4, 1965 2 Sheets-Sheet 2 I6 50 52 h 40 H; E 51 qm I V. 5' INVENTOR.

BY Vernon D. Es e Fig. 6 %q7L//MA4/g% ATTORNEYS United States Patent 3,503,333 RESISTANCE FUSE WIRE Vernon Dale Estes, Penrose, Colo. Continuation of application Ser. No. 492,399, Oct. 4, 1965. This application Dec. 6, 1967, Ser. No. 723,958

Int. Cl. F42]: 3/12 US. Cl. 102-48 2 Claims ABSTRACT 0F THE DISCLOSURE .The. disclosure is to a smalldiameter wire constituting This invention relates to fuses for igniting powder charges and the like, and more particularly to electric fuses. Accordingly, the invention will be hereinafter referred to as an electrical fuse, and also as a wire for electrical fuses, or a fuse wire.

This application is a continuation of my copending application, Ser. No. 492,399 filed Oct. 4, 1965, and now Patent No. 3,363,559.

The improved electrical fuse and fuse wire constituting the present invention was conceived and developed to meet the need for a better and more reliable means for igniting pyropulsive motors used in model rockets. Accordingly, the invention will be described for use with pyropulsive model rocket motors, although it is to be emphasized that such use is not at all restrictive and that other practical applications and uses for the fuse wire can be easily found.

The flying of model rockets has developed into a nationwide sport, especially among scientific-minded teenage boys, with its popularity ever increasing as the nation engages in more and more prototype rocket activities. However, from its inception, model rocket flying has created considerable concern as to safety because some hazards exist when rockets are not properly handled. To minimize the hazards involved, standardized motor units for model rockets have been developed and are made available to the public. These motors are cylindrical units, approximately three-fourth inch in diameter and three inches long. Carefully prepared loads of selected pyrogenic propellants constitute the fuel. This fuel will ordinarily burn at a controlled rate to create a high velocity propulsive jet which discharges through a nozzle at the trailing end of the motor to produce a substantial thrust.

Basically, the design of these rocket motors is similar to many other pyrotechnic devices, and it is a common practice to ignite the rocket motors with a short length of ordinary fuse thrust into the discharge end of the cylindrical motor unit, the same as any other pyrotechnic device would be ignited. However, this practice is not desirable because an individual must be close to the model rocket when it is ignited.

A better practice is to ignite the model rocket motors by remote means such as through an electrical system. For this purpose, several types of electrical fuses have been developed, a common type being a small-diameter, high-resistance Nichrorne wire which is inserted into the discharge end of a rocket motor. An electrical current heats the wire to a temperature sufficient to ignite fuel in ice the motor. It has been found, however, that such an arrangement is very awkward to fit into a toy rocket motor and requires comparatively large storage batteries. Other types of fuse wires which are available are similar to those used in electrical blasting caps. These fuse wires are essentially copper lead wires connected together by a short length of high-resistance Nichrome wire, or the like, which may be heated to high temperatures by an electrical current. Blasting cap fuse wires, however, were found to be generally too large and far too expensive for igniting a small rocket engine.

It follows that an object of the invention is to provide a novel and improved fuse wire for model rocket motors and the like which may be easily bent and fitted into a small socket in the end of a rocket motor, and which is adapted to generate an intense heat at only a selected short reach of the wire within the motor whenever an electric current is passed through it.

Another object of the invention is to provide an electric fuse wire which may be easily proportioned to provide an intense heat at only a short reach along the wire and thereby requires only a small electric voltage to produce the required heating current and which can be heated with only the limited output of a small dry cell battery.

Another object of the invention is to provide a novel and improved electric fuse wire adapted to generate heat at only a short reach along the wire, and which includes a covering of pyrogenic material enveloping this reach, of a type which is adapted to ignite and flare to produce an intensely hot flame to thereby assure ignition of the fuel in the rocket motor when the fuse is mounted in its discharge end.

Another object of the invention is to provide, in an electrical fuse wire having a high resistance reach between low resistances reaches, a novel and improved covering of pyrogenic material enveloping the high resistance reach, characterized by igniting when an electrical current heats the wire, but by being otherwise difficult to ignite, thus affording much greater safety in shipment and use in comparison with conventional match head ypes of fuse wire coverings of types used in blasting cap uses.

A further object of the invention is to provide an improved and simplified method of manufacturing a fuse wire stock of a type having reaches or segments of variable resistance, including a short reach of a high resistance between longer reaches of low resistance, and which may be cut into sections as individual fuse wire lengths.

A further object of the invention is to provide a novel and improved fuse wire which is especially useful where a point of intense heat is desired and where only a limited electrical current is available for the purpose.

Another object of the invention is to provide a novel and improved fuse wire capable of producing an intense heating action at a short reach at the center of the wire whenever an electrical current is passing through it, which may be made of wire of very small diameter, with the cost per unit being so low that an individual fuse wire can be easily provided each time a rocket motor is ignited, or an individual toy rocket motor to be equipped with its own fuse, and which is so constructed as to permit repeated use of a single wire if such becomes desirable.

With the foregoing and other objects in view, all of which more fully hereinafter appear, my invention comprises certain constructions, combinations, and arrangements of parts and elements as hereinafter described, defined in the appended claims and illustrated in preferred embodiment in the accompanying drawing, in which:

FIGURE 1 is a longitudinal sectional view of a conventional rocket motor illustrating the manner in which an ordinary fuse is fitted into the end of the motor to ignite the same.

FIGURE 2 is a longitudinal sectional view similar to FIG. 1, but illustrating one manner in which the improved electric fuse may be installed therein and illustrating further, in a somewhat diagrammatical manner, electrical wires connecting with the fuse and a portion of a battery connected with the wires to complete the circuit thereof.

FIGURE 3 is a longitudinal view of an unbent length of one form of the improved fuse wire as it will normally appear before being used.

FIGURE 4 is a fragmentary portion of the showing at FIG. 3 on a greatly enlarged scale and illustrated as being partially in section, as from the indicated line 4-4 at FIG. 3, to show'also one manner in" which the Wire may be coated.

FIGURE 5 is a transverse sectional View as taken from the indicated line 5-5 at FIG. 4, but on a further enlarged scale.

FIGURE 6 is a fragmentary portion, partially in section, similar to FIG. 4, but showing an alternate arrangement thereof.

FIGURE 7 is a fragmentary portion, similar to FIG. 4, but showing only the fuse wire itself, as before it is covered with insulation and before other elements are added to it.

FIGURE 8 is a transverse sectional'view as taken from the indicated line 88 at FIG. 7, but on a greatly enlarged scale and with indicating lines suggesting dimensions necessary to consider in the comparative proportioning of the components of the fuse wire.

FIGURE 9 is a fragmentary portion of a rocket motor, similar to the unit shown at FIG. 2, but on a greatly enlarged scale and being modified to incorporate the improved fuse wire into a motor, as a part thereof.

FIGURE 10 is a fragmentary portion of a bare fuse wire, similar to FIG. 7, but showing a related, but different invention, that of the application, Serial No. 492,399.

FIGURE 11 is a transverse sectional view as taken from the indicated line 1111 at FIG. 10, but on an enlarged scale.

FIGURE 12 is a fragmentary view of the wire illustrated at FIG. 10, but with a modified form of pyrogenous coating thereon shown in section.

FIGURE 13 is a reduced scale isometric view of a length of wire as it would appear when it is being manufactured into fuses according to the construction illustrated at FIG. 10.

FIGURE 14 is an isometric view, similar to FIG. 13, but illustrating the wire as it would appear when the fuses are substantially complete and with a portion of the wire being cut away from the remainder to form a completed fuse wire.

FIGURE 15 is a fragmentary portion of a bare fuse Wire, similar to FIG. 10, but showing an alternate embodiment of the related, but different invention.

FIGURE 16 is a transverse sectional view as taken from the indicated line 1616 at FIG. 15, but on an enlarged scale.

Referring more particularly to the drawing, the conventional rocket motor M. illustrated at FIG. 1, is formed as a cylindrical shell 10 having a restrictive nozzle 11 of refractory material at its discharge end. A propulsive powder charge 12 is packed within the body of the shell in a manner which causes a controlled burning action, rather than an explosive action, and the burning of the charge will eject gas from the discharge end of the unit whenever the charge is ignited. A special powder charge 13 may, or may not, be located at the leading end of this toy rocket to ignite when the propulsive charge burns out. Ignition of the propulsive charge may be by means of an ordinary fuse 14, and the powder charge at the nozzle opening can be shaped with a shallow socket 15, of comparatively small diameter to receive this fuse. This charge is ordinarily accessible only through the socket.

The present invention, an improved type of an electric fuse wire, is illustrated as a bimetal type fuse wire 20 at FIGS. 3 to 9 and in the related invention of the application Serial No. 492,399 as a necked-down type of fuse wire 40 at FIGS. 10 to 16. It is contemplated that either type will be used in the same manner. The improved electric fuse 20, or 40, may be conveniently provided as a straight, short length, as illustrated at FIG. 3, which will be folded and inserted into the socket 15 of a rocket motor, with each end of the fuse wire extending therefrom in a spread-apart arrangement so that shorting will not occur when the ends are connected is an electrical circuit. Each end of the wire'20 is connected to a suitable clip 21 of the circuit wire 22 of a power supply. In the usual arrangement, one circuit wire 22 is connected to a terminal 23 of a dry cell 24, and the other circuit wire 22 is preferably connected to the leg of a switch 25 which, in turn, is connected to the other terminal 23' of the battery. If desired, a small filler 26 may be inserted into the socket 15 to hold the wires apart and in position until the fuse ignites the powder charge.

The fuse wire 20, or 49, is formed with a high resistance portion at its center where it is bent and folded. The remainder of the wire, at each side of the high resistance portion, is of comparatively low resistance. Accordingly, an electrical current through the wirewill cause the high resistance portion to become heated without significantly heating the rest of the wire, and thus the high resistance portion forms a heating section at the center of each fuse wire.

The construction illustrated at FIGS. 3 to 9 provides for a wire having a high resistance core 30 encased in a low resistance sheath 31 at each side of the central heating section 32, a short, exposed reach of the core wire 30.

The core Wire 30 is of high-resistance metal such as Nichrome wire of the type commonly used in electrical heaters, and it is to be noted that although this core wire 30 extends through the length of the fuse wire 20, only the heating section 32 is effective, and the remainder performs no function other than that of holding the sheath 31. The electric current passes primarily through the low resistance sheath except at the heating section, and it is to be noted that the diameter of the core wire d the length of the section 32 and the diameter of the sheath d may be easily determined by a skilled electrical engineer, for any given voltage and power output of an electrical source to produce a selected temperature at the heating section 32 without overheating the remainder of the wire.

It is also to be noted that where only a low-voltage, low-output dry cell battery is available, the wires must be very small in diameter, making for very low cost fuses. For example, it was found that a No. 30 Nichrome wire having a diameter of 0.01003-inch would be suitable for the purpose at hand where only small 3- and 6-volt batteries were available, and that such wire could even be used for l /z-volt batteries if desired. The cost per fuse is so small as to permit each fuse to be a single-use, throwaway item.

The sheath 31 is preferably of copper or like material having a low electrical resistivity, and it was found that if a copper sheath having a thickness of 0.0005-inch were used about the No. 30 Nichrome wire, the resistance of the combined core wire and sheath would be reduced to the point where the small batteries mentioned above would effectively raise the temperature of the heating section 32. This unsheathed heating section 32 of the core could be heated until it was practically white-hot with out significantly overheating the remaining sheathed portions of the Wire.

Although the heating section 32 may be heated to a temperature sufiicient to ignite the fuel of the model rocket motor, it is desirable to enclose the heating section 32 in a covering or bead 33 formed of a pyrogenous substance which will flare with an intense flame when the exposed core wire 30 becomes hot responsive to an electrical current through the wire. Suitable materials for the manufacture of such pyrogenous beads will be hereinafter described.

To complete the fuse, the conductive portions of the wire, at each side of the heating section 32, may be pro tected against shorting by a suitable insulation 34. This insulation 34, when used, is preferably a coating of enamel of the type commonly used where low voltage electrical insulation is required. However, a more elaborate insulation may be used, and the enamel coating 34 may be replaced or reinforced by rubber insulation 35, as illustrated at FIG. 6. It is to be noted that the enamel coating may cover the heating section 32 or the bead 33, although not illustrated as doing so.

In using the small-diameter core wire 30, it was found that the copper sheath 31 could be applied to the core wire 30 by electrodeposition in a somewhat conventional manner. It is to be noted that a plating operation to effect the electrodeposition of copper, on standard Nichrome electrical resistance wire required first an activation step in a bath of hydrochloric acid and copper sulphate to eliminate an oxide coating normally found on the-Nichrome wire. Next, a very thin coating or flash of copper had to be deposited on the Nichrome wire by a chemical action to eliminate a thin oxide film. However once the flash coat had been applied, an ordinary copper plating procedure would then be used, with a copper supply bar being the electrical anode, and the wire the cathode spaced a distance of approximately one inch from the anode. With the current density of approximately 90 amperes per square foot, somewhat in excess of that ordinarily recommended copper plating, a five minute interval was required in order to obtain a half mill thickness of copper on the 30gage Nichrome wire.

The steps of copper plating this wire may be accomplished in any manner advantageous for production as, for example, by copper plating a number of individual wires at a time, or preferably by copper plating as a continuous process one or more lengths of wire which continuously move through the solution at a slow, steady rate, and which are subsequently cut to length.

A further step involves the mechanical masking of the Nichrome wire where the heating sections 32 are located to prevent copper from being deposited at those sections. It was found that a natural latex or any like rubbery acid-resistant material could be applied to each heating section 32 at the proper location of individual wires or at selected spacings along the reach of a continuous wire. Also, it was found that ignition beads 33 may be spotted on the wire 30 prior to the electroplating operation when the beads were of a material which was acid-proof and would remain on the wire and effectively prevent the deposit of any copper at the heating sections 32.

The modified construction of a fuse wire 40 of the related invention of the application Ser. No. 492,399, illustrated at FIGS. to 12, or of a fuse wire 50 at FIGS. and 16, provides for a central, high resistance heating section 42, and 52, at the center of a wire which is formed by reducing the area of the section, either by cutting away a portion of the wire, as illustrated at FIGS. 10 to 12, or by swadging or necking thew ire, as illustrated at FIGS. 15 and 16. Such a wire 40, or 50, may be of almost any metal, preferably steel or Nichrome, and although the wire itself may be comparatively small with high resistance, the increased resistance at the neckeddown section will nevertheless form a region of intense heat whenever an electric current is passed through it.

The construction illustrated specifically at FIGS. 10 to 12 contemplates the use of small lengths of small-diameter wire as the fuse 40, the length of the wire to be used and its resistivity determining the comparative proportions of the components. For example, it was found that with a two-inch length of 28-gage Nichrome fuse wire 40, having a diameter of 0.0126 inch, a small dry cell storage battery could be used to heat the reduced-area heating section 42. The reduced-area heating section 42 of the fuse 40 is formed by cutting away one side of the wire, leaving a narrow segment strip 41 at the uncut side. In the two-inch, 28-gage fuse wire selected, it was established that the length of this central section could be approximately Ml-lIlCh. The cross sectional area of the reduced-area portion 21 should be approximately A the area of the undisturbed section of the wire itself. However, the actual proportions of the wire and the reducedarea portion can be varied and established for any particular wire by simple tests. To complete the fuse 40, a pyrogenous coating 43 may be placed over the reduced diameter portion 41 forming the heating section 42, however, instead of using a bead-like structure, as illustrated at FIG. 3, the coating 43 may be distributed along the heating section more nearly as a short sheath, as shown at FIG. 12. When used in this manner, the coating may serve as a short electrical insulator.

The manufacture of a fuse 40 by cutting away a portion of the wire to form the reduced-area central section 42 may be by comparatively simple, rapid and low-cost operations. It is preferably manufactured as a continuous process, and FIG. 13 illustrates a continuous length of wire 45 which may be fed through a milling machine having a cutter which is adapted to intermittently engage the wire, at selected spacings, as the wire moves through the machine. For example, the wire 45 may be fed about a cammed head in the milling machine which rotates with the movement of the wire and which exposes sections of the wire to a milling cutter mounted alongside the cam. With apparatus such as this, or with similar apparatus, the wire may be fed through the apparatus to be discharged therefrom at a rapid rate, and the cuts 46 will appear at regular intervals, the cuts 46 being the reducedarea portions 42 of the finished fuse wire units.

The wire is next passed through a coating machine which will apply pyrogenous coatings 47 over the reduced-area sections 46 to provide for a final unit 40 which may be cut from an end of a wire 45, as illustrated at FIG. 14. In supplying fuses 40, it may even be desirable to provide a length of wire 45 having a number of fuse sections 40 on it for easy shipment, care and storage. The individual fuses 40 may then be cut from this supply wire as needed.

The construction illustrated at FIGS. 15 and 16 provides for a fuse wire 50 which is functionally the same as the fuse wire illustrated at FIGS. 10 to 12, including a short, heating section 52 to a reduced-area. However, the heating section 52 is formed by swadging, or a similar operation, where the short section of the wire is actually deformed under pressure. It is to be noted that the swadging operation will produce a more uniform cross section of wire, and that such operation may be very rapid. However, it is to be further noted that such reducing of the section in this manner requires a ductile wire, for otherwise, the working of the wire would render it brittle at this reduced-area section.

A pyrogenous starter, such as a bead 33, or sheath coating 43, though desirable, is not absolutely necessary for starting a model rocket motor. Ordinary black powder, such as used in model rocket motors can be ignited by a wire reaching a temperature not greatly in excess of 1000 degrees F. if the wire section 32 is contacting the powder. Likewise, the sections 41 and 51 of FIGS. 10 and 15 may heat in the same manner. It was found preferable, however, to provide an ignition bead 33 of pyrogen-type compound which tightly embraces the heating section of the wire and which will ignite at temperatures in the range of approximately 750 to 1000 degrees F. The head will flare to a much higher temperature, and this flare will practically assure the ignition of the powder in the rocket motor even when the bead within the socket 15 is not actually touching the powder.

It was found that a suitable bead could be prepared by using ingredients similar to those which make up the head of an ordinary safety match, such as that disclosed in US. Patent No. 2,103,695. However, a different binder is needed especially where the bead is to be used as a masking element in the electroplating operation and where it must be water-proof and acid-proof. An ideal binder is a nitrocellulose laquer or like inflammable laquer which adds fuel to the flare when the bead is ignited.

The blend for forming such beads includes iron powder, potassium chlorate, manganese dioxide, and a binder. One formulation of ingredients is as follows:

Parts Iron powder 28 Potassium chlorate 2.8 Manganese dioxide 1 The binder for these materials included a conventional nitrocellulose laquer which was varied somewhat, and one formulation was made by using 16- parts of 21 percent Komac brushing lacquer. Another formulation used 48 parts of 21 percent Komac lacquer, and this formulation was sufficiently tough to permit straight wire sections, such as illustrated at FIG. 3, to be bent to the desired hairpin shape without significantly disturbing the beads.

A third compound found to be especially effective consists of:

Parts Iron powder 70 Potassium chloride 110 Manganese dioxide 3 and a binder consisting of flammable synthetic resins known to the trade as:

Parts Epon 828 Thiokol Lp-3 50 MEK thinner 27 Following the application of the ignition bead 3 3 at each heating section 32, the insulation coating 34 may be applied in any suitable manner, as by dipping or spraying. Should the ignition bead be covered by an insulative coating of enamel, no adverse effect is encountered, for the enamel will not deter the flare action of the bead. It is to be noted, however, that if a dipping operation is used, wherein the beads are also covered with enamel, there must be a comparatively rapid operation if the thinner in the enamel is of a type which also dissolves the binder material of the bead.

In the construction illustrated at FIG. 9, the fuse wire 20 is illustrated as being embedded in the discharge mouth of a rocket motor. As illustrated, this is a simple operation, where the wire is folded at the heating section with each leg thereof lying alongside, but not contacting the other leg. Insertion into the rocket motor at the time of manufacture of the motor is a simple matter, for the shell is ordinarily loaded with fuel, usually black powder, by compacting the fuel into the shell 10 from the forward end and against the nozzle 11, with molds holding the discharge end and forming the short fuse socket 15. The same mold may be slightly modified to hold the fuse wire before insertion of fuel and so that the wire is partially, but not completely, embedded in the fuel powder charge 12 in a socket 15', illustrated at FIG. 9.

The wire itself, when properly proportioned as described, can be easily heated at the heating section to a temperature which will ignite a conventional powder charge in a rocket motor. However, if desired, the pyrogenous head 33 may be included, as illustrated, to provide an arrangement which will properly ignite in every instance, being almost completely fool proof.

The manner in which the wire and bead may be embedded into the end of the motor may be varied. Preferably, it should be partially embedded so that gas from the ignited fuel may commence to be discharged the instant ignition commences. If completely embedded, the depth of embedment from the walls of the socket 15' should be very slight to permit the gases to break away the confining wall of fuel and permit movement of gas from the motor. A further alternative, not shown, is to place the fuse wire in the socket 15 and secure it in position by a loosely packed charge of powder. If desired, the discharge end of the rocket motor may then be closed off by simple paper washer or the like so that only the ends of the fuse wire project from the shell. As a final product, the fuse structure, so described, provides for a maximum safety and reliability of operation when handled by amateurs.

What is claimed is:

1. A resistance fuse wire for ignition of an ignitable charge comprising a continuous wire core of high resistance type conductive material adapted to be folded into a narrow U-shaped section having straight portions and an interconnecting curved portion for insertion into a charge whereby ignition of said charge will occur upon the passage of a current of sufficient intensity therethrough; spaced sheath means of low resistance type metal surrounding and in conductive contact with the straight portions of said U-shaped section of wire; a pyrogenous coating completely surrounding the exposed portion of said wire between the spaced ends of said sheath means and extending over and completely surrounding the terminal ends of said spaced sheath means; said core wire being of Nichrome and said sheath means being of copper.

2. An article of manufacture for forming a plurality of spaced interconnected fuse wires which when separated may each be folded into a narrow U-shaped section having straight portions and an interconnected curved portion for insertion into an ignitable charge whereby ignition of said charge will occur upon the passage of a current of suflicient intensity therethrough which comprises a con tinuous core wire of high resistance type conductive material; spaced sheath means of low resistance type metal surrounding and in conductive contact with the core wire; and pyrogenous coating means completely surrounding each exposed portion of said core wire between adjacent ends of said sheath means and extending over and completely surrounding the adjacent ends of said sheath means; said core wire being of Nichrome and said sheath means being of copper.

References Cited UNITED STATES PATENTS 2,429,490 10/1947 Scherrer. 2,801,585 8/1957 Smith. 2,960,933 11/1960 Scherrer 102-28 3,363,559 1/1968 Estes 102-28 FOREIGN PATENTS 555,769 9/ 1943 Great Britain.

VERLIN R. PENDEGRASS, Primary Examiner 

